Off-site Cooper pairing in strong electron-phonon interacting systems

Abstract

We have developed a new variational treatment to deal with the superconductivity properties of systems with strong electron-phonon interactions by introducing a ‘‘squeeze’’ transformation for phonon operators. It is shown that as long as the renormalized intersite electron correlation is attractive, a new type of Cooper pairing occurs whether the renormalized on-site electron correlation is repulsive or atttractive. The transformed electron-phonon interaction gives rise to the squeezed polarons which are, in general, different from the ordinary polarons. We have found from this theory that: (1) the squeezed polaronic state corresponds to a stable minimum of the total energy; (2) the effective mass of the squeezed polaron is much smaller than that of ordinary polarons; (3) both the gap equation and the expression for the critical temperature Tc in squeezed polaronic states are different from those in ordinary polaronic states as well as from those in BCS theory; and (4) high Tc is obtained for simple cubic and body centered cubic lattice structures with the electron concentration n=1.